A re-entrant shape is a form which is wider at the inside than it is at the entrance (e.g., a dovetail joint). Turbine components, such as jet engine, rotors, compressor blade assembly, typically employ re-entrant shaped slots in the turbine disks. The re-entrant shape is used to hold or retain turbine blades around the periphery of turbine disks. Mechanical slides, T-slots to clamp parts on a machine table also use such re-entrant shaped slots.
This type of form cannot generally be created by grinding with a large diameter wheel operated perpendicular to the surface of the part because it would be impossible for the wheel to enter the wider part of the form without removing the narrower part of the form. Typically, broaching or milling has been used in the aerospace industry to produce such a complex shape. Broaching a re-entrant shape, however, is costly partly due to high tooling costs, such as expensive machinery, set-up costs, tooling regrinding costs and slow material removal rates. One of the traditional advantages of broaching over grinding is very low heat generation during the process, which results in good surface integrity. However, this requires frequent tool changes and re-sharpening of dulled cutting edges, which is cost and time intensive. Milling processes are generally very slow, especially in machining difficult-to-machine materials, such as Inconel™ nickel alloy, which is typically used for re-entrant shaped turbine disks of aeroengines. Although high speed milling can be conducted to achieve high efficiency, under such high speeds, fracture of the cutting edge of milling tools commonly occurs, often leading to imbalance, tool fracture and failure of the process.
Conventional broaching, machining and milling processes employ an oil coolant to avoid thermal damage and residual stress to the workpiece. Prior art grinding processes developed to replace machining processes, such as the grinding process described in U.S. Pat. No. 6,883,234 B2, also employ an oil coolant both during formation of slots and during formation of complex shapes. Environmental considerations have led operators to search for processes wherein a water-based coolant can be used in lieu of an oil coolant, while still avoiding thermal damage and residual stress to the workpiece.
Therefore, there is a need to develop new grinding methods to form a complex shape, such as a re-entrant shape, in a workpiece overcoming or minimizing one or more of the shortcomings associated with conventional processes, such as broaching, machining and/or milling processes.